Mendelian laws of inheritance

Photo by: iQoncept

Mendelian laws of inheritance are statements about the way certain
characteristics are transmitted from one generation to another in an
organism.
The laws were derived by the Austrian monk Gregor Mendel
(1822–1884) based on experiments he conducted in the period from
about 1857 to 1865. For his experiments, Mendel used ordinary pea plants.
Among the traits that Mendel studied were the color of a plant's
flowers, their location on the plant, the shape and color of pea pods, the
shape and color of seeds, and the length of plant stems.

Mendel's approach was to transfer pollen (which contains male sex
cells) from the stamen (the male reproductive organ) of one pea plant to
the pistil (female reproductive organ) of a second pea plant. As a simple
example of this kind of experiment, suppose that one takes pollen from a
pea plant with red flowers and uses it to fertilize a pea plant with white
flowers. What Mendel wanted to know is what color the flowers would be in
the offspring of these two plants. In a second series of experiments,
Mendel studied the changes that occurred in the second generation. That
is, suppose two offspring of the red/white mating ("cross")
are themselves mated. What color will the flowers be in this second
generation of plants? As a result of these experiments, Mendel was able to
state three generalizations about the way characteristics are transmitted
from one generation to the next in pea plants.

Words to Know

Allele:
One of two or more forms a gene may take.

Dominant:
An allele whose expression overpowers the effect of a second form of
the same gene.

Gamete:
A reproductive cell.

Heterozygous:
A condition in which two alleles for a given gene are different from
each other.

Homozygous:
A condition in which two alleles for a given gene are the same.

Recessive:
An allele whose effects are concealed in offspring by the dominant
allele in the pair.

Terminology

Before reviewing these three laws, it will be helpful to define some of
the terms used in talking about Mendel's laws of inheritance. Most
of
these terms were invented not by Mendel, but by biologists some years
after his research was originally published.

Genes are the units in which characteristics are passed from one
generation to the next. For example, a plant with red flowers must carry a
gene for that characteristic.

A gene for any given characteristic may occur in one of two forms, called
the alleles (pronounced uh-LEELZ) of that gene. For example, the gene for
color in pea plants can occur in the form (allele) for a white flower or
in the form (allele) for a red color.

The first step that takes place in reproduction is for the sex cells in
plants to divide into two halves, called gametes. The next step is for the
gametes from the male plant to combine with the gametes of the female
plant to produce a fertilized egg. That fertilized egg is called a zygote.
A zygote contains genetic information from both parents.

For example, a zygote might contain one allele for white flowers and one
allele for red flowers. The plant that develops from that zygote would
said to be heterozygous for that trait since its gene for flower color has
two different alleles. If the zygote contains a gene with two identical
alleles, it is said to be homozygous.

Mendel's Law of Segregation.
(Reproduced by permission of

The Gale Group

.)

Mendel's laws

Mendel's law of segregation describes what happens to the alleles
that make up a gene during formation of gametes. For example, suppose that
a pea plant contains a gene for flower color in which both alleles code
for red. One way to represent that condition is to write RR, which
indicates that both alleles (R and R) code for the color red. Another gene
might have a different combination of alleles, as in Rr. In this case, the
symbol R stands for red color and the r for "not red" or, in
this case, white. Mendel's law of segregation says that the alleles
that make up a gene separate from each other, or segregate, during the
formation of gametes. That fact can be represented by simple equations,
such as:

RR → R + R or Rr → R + r

Mendel's second law is called the law of independent assortment.
That law refers to the fact that any plant contains many different kinds
of genes. One gene determines flower color, a second gene determines
length of stem, a third gene determines shape of pea pods, and so on.
Mendel discovered that the way in which alleles from different genes
separate and then recombine is unconnected to other genes. That is,
suppose that a plant contains genes for color (RR) and for shape of pod
(TT). Then Mendel's second law says that the two genes will
segregate independently, as:

RR → R + R and TT → T + T

Mendel's third law deals with the matter of dominance. Suppose that
a gene contains an allele for red color (R) and an allele for white color
(r). What will be the color of the flowers produced on this plant?
Mendel's answer was that in every pair of alleles, one is more
likely to be expressed than the other. In other words, one allele is
dominant and the other allele is recessive. In the example of an Rr gene,
the flowers produced will be red because the allele R is dominant over the
allele r.

Predicting traits

The application of Mendel's three laws makes it possible to predict
the characteristics of offspring produced by parents of known genetic
composition. The picture on page 1248, for example, shows the cross
between a sweet pea plant with red flowers (RR) and one with white flowers
(rr). Notice that the genes from the two parents will segregate to produce
the corresponding alleles:

RR → R + R and rr → r + r

There are, then, four ways in which those alleles can recombine, as shown
in the same picture. However, all four combinations produce
the same result: R + r → Rr. In every case, the gene formed will
consist of an allele for red (R) and an allele for "not red"
(r).

The drawing at the right in the picture on page 1248 shows what happens
when two plants from the first generation are crossed with each other.
Again, the alleles of each plant separate from each other:

Rr → R + r

Again, the alleles can recombine in four ways. In this case, however, the
results are different from those in the first generation. The possible
results of these combinations are two Rr combinations, one RR combination,
and one rr combination. Since R is dominant over r, three of the four
combinations will produce plants with red flowers and one (the rr option)
will product plants with non-red (white) flowers.

Biologists have discovered that Mendel's laws are simplifications
of processes that are sometimes much more complex than the examples given
here. However, those laws still form an important foundation for the
science of genetics.

User Contributions:

i need help on this question its my homework please help!!!! one trait in pea plants is the color of their pea pods.the gene for the trait has two alleles. the green allele (g) is dominant , and the yellow allele is recessive (g). the punnett square shows a cross between two parents each with a dominant allele and recessive allele. filling in the punnent squre will show you what traits their offspring will have.

HELP PLEASE! ok 1. For Mednel's law of segregation to occur, the alleles must be where? a. at different spots on the same chromosome b. at the same spot on homologous chromoseomes. c. on nonhomolgous chromosomes. do at the same spot on the same chromosome. or e. none of that above

I am stuck on some homework qestions!!! I'm trying to figure out if a woman who isn't colorblind but has an allele for color blindness reproduces with a man who has normal vision. What is the chance that they will have a colorblind daughter??? I have looked all in my books but I'm so desprate.

I have another question too. I have never used this site. I dont know if you can ask two questions.

But my other ? is: A recessive allele t is responsible for a condition called distonia. A man who has this condition marries a woman who doesn't. One of their 4 children has a condition. What are the possible genotypes of the man and the woman?

THIS ARTICLE IS NICE, THANKS FOR THE GOOD WORK YOUR HAVE DONE TO ENSURE THAT MOST PEOPLE GET INFORMATION WHEN DEALING WITH THE GENETICS ISSUE BUT IF YOU COULD HAVE USED MORE DIAGRAMS IT COULD HAVE BEEN PERFECT. THANKS AND KEEP IT UP.

Very itisreentng example of a cause of extinction. Do you think this lack of genetic diversity could also lead to mutations? And if so, maybe this could revive the cheetah population.This lack of diversity will prove to be a great complication if there is ever a change in the cheetahs' environment.

Hi! This is a very good article. It helps me understand Mendel's Law of Inheritance but there is not much on the second law(Law of Independent Assortment). Can you please ellaborate more on the secod law this.

He use characteristis for .e.g. colour, shape and texture of the offspring. He used the larg sampling size wgich gave him credibility to his data. He also look through several successive generations of his pea plant.

This is the most intresting article I have ever read because it help me a lots to learn much about Gregor Mendel's Laws. Thank you for writing this article for students to learn and understanding more Mendel's Laws of Inheritance.

hai po good evening i would like to have a favor from you all the informations of mendels law of inheritance,illustration of it,the punnet square the aprental F1 and F2 all about that...for my project only.

I really wanna appreciate Science Clarified. Gregor's experiments still remain a shock to people but thanks to you guys, it has been made easier to understand. It is not known to people that there is a third law of genetics and i hope people will find out just like i have done